Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a probe or “sonde” is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.
In these and other logging environments, it is desirable to construct an image of the borehole wall. Among other things, such images reveal the fine-scale structure of the penetrated formations. The fine-scale structure includes stratifications such as shale/sand sequences, fractures, and non-homogeneities caused by irregular cementation and variations in pore size. Orientations of fractures and strata can also be identified, enabling more accurate reservoir flow modeling.
Borehole wall imaging can be accomplished in a number of ways, including ultrasonic imaging, micro-resistivity imaging, or azimuthally-sensitive varieties of laterologs, induction logs, nuclear magnetic resonance logs, gamma-ray logs, and other azimuthally-sensitive logs. In many cases, the image logs may appear accurate while failing to reflect the actual properties of the borehole wall. Such errors can result from erratic tool motion or poor wall contact during the logging process. Conversely, image logs may appear “noisy” (a symptom of erratic tool motion or poor wall contact) when in fact they accurately reflect the actual properties of the borehole wall. In the past, log quality judgments have been performed by experts using a subjective, error-prone approach.
The drawings show illustrative invention embodiments that will be described in detail. However, the description and accompanying drawings are not intended to limit the invention to the illustrative embodiments, but to the contrary, the intention is to disclose and protect all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.